President Donald Trump confers with Chairman of the Joint Chiefs of Staff Joseph Dunford (right) during a meeting with military leaders in the Cabinet Room in October. (Win McNamee/Getty Images file photo)

Updated 12/26/18 | This week was a stormy one for American national security. But it is likely to be only a taste of things to come.

The disclosure that President Donald Trump plans to withdraw thousands of U.S. troops from Syria and possibly also Afghanistan, followed by Defense Secretary James Mattis’ resignation in protest of those moves and more, jangled nerves in Washington.

But even if Trump does not do anything else unpredictable in the months ahead — an admittedly unlikely prospect — 2019 will see still more tumultuous events as a new Congress tries to figure out what comes next.

First will probably come an intense period of hearings and other forms of scrutiny of the withdrawal decisions, though the net effect of this oversight will probably be negligible. Then will arrive in the Senate a new Defense secretary nomination for a person culled from a shrunken pool of second-tier contenders willing to work for Trump, followed by nominees to replace virtually the entire Joint Chiefs of Staff, whose terms are almost all up in 2019.

Meanwhile, a fiscal 2020 budget debate falls in the middle of all this, and it will feature a fight to move many billions previously tabbed for spending on overseas conflicts toward military hardware.

If all that is settled, the problems may only begin, because global crises will assuredly erupt in the middle of this mostly self-inflicted upheaval.

Here are five points to remember about what lies ahead.

1. Armed Services members from both parties will keep condemning the withdrawals but probably won’t do much of substance.

The leaders of those congressional committees, to varying degrees, are concerned about the troop retreats.

Washington Democrat Adam Smith, the incoming House Armed Services chairman, is working with Texas Republican Mac Thornberry, the outgoing chairman, on a way to resist Trump's proposals.

“In a bipartisan way, we’re going to push back on it and try to get the president to change his mind,” Smith said Thursday, in a reference to the Syria withdrawal. “Mr. Thornberry and I are talking and trying to figure out how we should approach it.”

Many Democrats who cheered former President Barack Obama’s timetables for withdrawal from war zones are newfound students of the stay-the-course school, while Republicans calling for continued engagement at least can claim consistency. Even so, some elements of both parties are pleased with the withdrawals, even if some Democrats won’t say so for political reasons.

Regardless of the politics, the Armed Services panels, whose members on both sides are more hawkish than their colleagues in Congress as a whole, will cast the withdrawals in a harsh light.

But with the holidays upon us and a reorganization and transition to the new Congress set to occur in January, there is not much time for members to conduct war oversight.

Plus, the president apparently wants to bring troops home within about a month. That, too, limits the time available. And the odds of Congress creating more time for scrutiny by forcing the commander in chief to slow the withdrawals appear low as of today. Most fundamentally, Congress is historically averse to reining in presidents on foreign policy and military affairs, and that trend should continue.

Besides hearings, “the most we might see is a resolution that we should stay engaged,” said Thomas Spoehr, a retired Army lieutenant general who is an analyst with the conservative Heritage Foundation.

2) Mattis’ replacement will likely be a second-stringer and will face a grilling in the Senate.

Trump’s pattern has been to turn to retired generals to fill senior civilian roles in his administration. But some of the ex-officers he reportedly considered in late 2016 and early 2017 for the Defense post and other top jobs have become critics of him or of his policies in general or of the withdrawals in particular. Retired Army four-star Gens. Jack Keane and David Petraeus fall into that category.

More broadly, the most qualified people will be wary of working for a president whose style and beliefs are counter to the military’s traditional values, as expressed by Mattis in his resignation letter. To wit, Trump made decisions impulsively and with little consultation with aides, allies or lawmakers. Sometimes those decisions have changed from day to day. And he has tried to turn allies into enemies and foes into pals. None of that sits well with most people in the pool of potential Pentagon chiefs.

Despite those limitations, Trump will find someone for the job. It may be a former senator whose nomination may go more smoothly as a result of his connections — someone like Jim Talent, R-Mo. Or he may reach to the Pentagon’s bench and tap someone like the telegenic and talented Heather Wilson, the Air Force secretary.

Spoehr of the Heritage Foundation says that, while the nominee may not be a retired general this time, the personality Trump is drawn to is the “warrior in a necktie,” meaning someone who can talk tough and be charismatic.

Whoever it is, even a former senator, will face what could be one of the most intense interrogations ever for a Defense secretary nominee, given the swirling questions about Trump’s policies and practices. Even Senate Majority Leader Mitch McConnell, R-Ky., the embodiment of party loyalty and the man who will oversee that nomination, issued a statement Thursday aligning himself with Mattis’ traditional security values including standing squarely with allies and against foes such as Russia and China.

“So I was sorry to learn that Secretary Mattis, who shares those clear principles, will soon depart the administration,” McConnell said in a statement. “But I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership.”

3) It’s not just Mattis. Nearly the entire top rung of U.S. military leadership will be new next year.

Trump has already tapped Army Chief of Staff Gen. Mark Milley to replace Marine Corps Gen. Joseph Dunford as chairman of the Joint Chiefs of Staff, even though Dunford’s term does not expire until the summer of 2019.

In addition to finding a new Army chief to take Milley’s old seat, Trump will need to replace the vice chairman of the Joint Chiefs of Staff and the top officers in the Navy and Marine Corps. Only the Air Force and National Guard Bureau chiefs have terms that extend into 2020.

There is some question as to what sort of officer would want to serve under Trump. But there will be candidates who — like Mattis was for a while — are people who want to serve despite the president, not because of him, and who may even believe they can shape how Trump behaves.

Sen. Jack Reed of Rhode Island, the top Democrat on Armed Services, told reporters this fall that all that turnover is “not going to be good for national defense."

4) The fiscal 2020 budget is set to land in Congress in February on the seam of the Defense secretary transition, complicating both processes.

Mattis had planned to stay in office through February, partly to usher the budget request to Congress that month. Now Trump wants Mattis out by New Years Day and has tapped Deputy Secretary Patrick Shanahan as the acting Defense boss.

The budget request generally arrives in Congress the first week of February. We’ll see if that schedule holds, given the leadership changes and the revisions that may be needed in the Overseas Contingency Operations budget because of the planned troop withdrawals.

The total or “top line” for defense programs in the Pentagon and other agencies that the administration may request in fiscal 2020 has reportedly fluctuated in recent weeks — from $733 billion down to $700 billion and up again all the way to $750 billion. Mattis was a champion of as large a defense budget as possible. With Mattis gone, might Trump bring the top line down again before submitting the request to Congress?

The budget was already upended this week by the likely reduction in billions of dollars for maintaining troop operations overseas.

The costs in fiscal 2019 for Afghanistan and Syria were $46 billion and $19 billion, respectively, says Roman Schweizer, a defense market analyst with the Cowen Washington Research Group, an investment firm, in a Thursday note to investors.

So the overseas operations and maintenance account in fiscal 2020 could drop by billions of dollars as a result of the extraction of forces.

To reflect the new reality, the top line of the overall Pentagon Overseas Contingency Operations account could just go down.

On the other hand, some or all of that money could be shifted to weapons accounts, which were already under pressure because they are easier for budget cutters in the Pentagon to rein in than are personnel and installations programs.

“We believe this could provide an opportunity to shift money from personnel and operations & maintenance to advanced research and development and procurement focused at China and Russia,” Schweizer wrote.

Against this backdrop, the debate over how much should be spent in the overseas budget on programs unrelated to war may take on a sharper focus this year. The administration wants to be more disciplined about that. But it will be as tempting as ever for Washington to use those billions in war funds for programs that benefit lawmakers’ constituencies and campaign contributors.

5) The world gets a vote on what troubles will be heaped onto this already muddled scenario.

First, there is the question of fallout from the withdrawal decisions themselves. Inside Syria, how will Turkey, Iran, Russia, the Kurds and the Islamic State group react to the vacuum left by U.S. forces, and how will the situation there affect Israel?

Thornberry, the House Armed Services chairman, said in a statement Friday he was “deeply disturbed” about the troop withdrawal from Afghanistan. In an interview the previous day, prior to Mattis' departure but after the Syria withdrawal was announced, he said he was pessimistic about the fallout in Syria.

“We’ve seen what happens when the U.S. abruptly withdraws from some place, and it’s not good,” Thornberry said.

The implications of this week’s moves are broader than any particular war zone or region.

How will the perception of an America that is shrinking from engagement and from previous commitments affect other countries and non-state fighting forces? As for friends, will they trust Washington less? As for foes, will they be emboldened? What will Russia do in Ukraine and China in the South China Sea?

"If the fans of this move are Russia, Syria, ISIS and China, you've got to wonder if this is the right move," said Spoehr.

On Thursday, it was instructive (but not much discussed) when, as Washington dealt with Trump-induced national security issues, North Korea’s leaders said they would not give up their nuclear weapons until the United States no longer poses a nuclear threat to Pyongyang. That was a crisis that Trump had claimed to have essentially solved — but every expert knew that was not true.

Then there are the coming flashpoints in places with names that seemed in the past — like Mogadishu in Somalia where an intense U.S. aerial bombing campaign is ongoing. And then there's Sarajevo and the Balkans, where tensions are rising again as Russia tries to pull the former Yugoslavian republics back to its orbit, or the Sea of Azov and Russian-Ukrainian tensions there.

It is not known where the crises of 2019 will occur, but it is certain that they will.